Process and arrangement for measuring a magnetic field using the faraday effect with compensation for variations in intensity and temperature effects

ABSTRACT

Two light signals pass through a series connection of a first multimode optical fiber, a first polarizer, a Faraday sensor device, a second polarizer an a second multimode optical fiber in opposite directions. The polarization axes of the two polarizers are set at a polarizer angle η or θ to the natural axis of the linear birefringence in the sensor device with cos(2η+2θ)=-2/3. The measuring signal is derived as the quotient of two linear functions of the light intensities of the two light signals after passing through the series connection.

FIELD OF THE INVENTION

The present invention concerns a process and an arrangement formeasuring a magnetic field.

BACKGROUND INFORMATION

Optical measuring arrangements and methods of measuring a magnetic fieldutilizing the magneto-optic Faraday effect are known. The Faraday effectis defined as the rotation of the plane of polarization of linearlypolarized light as a function of a magnetic field. The angle of rotationis proportional to the path integral over the magnetic field along thepath traveled by the light with the Verdet constant as a proportionalityconstant. The Verdet constant depends in general on the material, thetemperature, and the wavelength. To measure the magnetic field, aFaraday sensor device made of an optically transparent material such asglass is arranged in the magnetic field. The magnetic field causes theplane of polarization of linearly polarized light passed through theFaraday sensor device to rotate by an angle of rotation that can beanalyzed for a measuring signal. Such magneto-optical measuring methodsand arrangements are known for use in measuring electric currents. TheFaraday sensor device is placed near a current conductor and detects themagnetic field generated by the current in the conductor. The Faradaysensor device generally surrounds the current conductor, so themeasuring light travels around the current conductor in a closed path.In this case, the value of the angle of rotation is in goodapproximation directly proportional to the amplitude of the current tobe measured. The Faraday sensor device may be designed as a solid glassring around the current conductor or it may surround the currentconductor in the form of a measuring winding consisting of an opticalfiber (fiber coil) with at least one spire.

Advantages of these magneto-optical measuring arrangements and methods,in comparison with traditional inductive current transformers, includeelectrical isolation and insensitivity to electromagnetic disturbance.In the use of magneto-optic current transformers, however, problems areencountered due to the effects of mechanical vibrations on the sensordevice and the optical leads, which can cause changes in intensity thatfalsify the measurement, as well as the effects of changes intemperature, for example, in the sensor device.

To reduce the effects of vibration on the measurement, it is known thattwo oppositely directed light signals, i.e., light signals propagatingin opposite directions, can be transmitted through a Faraday sensordevice. This known measure is based on the idea that the linearbirefringences experienced by the two light signals along their commonpath due to vibrations as a reciprocal effect of the non-reciprocalFaraday effect can can be distinguished using suitable signalprocessing.

In a first known embodiment, two oppositely directed, linearly polarizedlight signals are transmitted through an optical fiber coil serving as aFaraday sensor device surrounding a current conductor. A twisted fiberor a spun hi-bi fiber (a high-birefringence fiber twisted during thedrawing process) is provided as the optical fiber for the fiber coil. Inaddition to the Faraday effect, the optical fiber also has a circularbirefringence that is high in comparison with the Faraday effect. Afterpassing through the sensor device, each of the two light signals isbroken down by a polarizing beam splitter into two components polarizednormally to one another. A measuring signal corresponding essentially tothe quotient of the Faraday measuring angle and the circularbirefringence of the fiber, which is thus independent of the linearbirefringence in the optical fiber, is derived by signal processing froma total of four light components. The resulting measuring signal is thuslargely free of temperature-induced linear birefringence in the sensordevice, but the measuring signal still depends on temperature because ofthe temperature-dependence of the circular birefringence of the fiber.In this known embodiment, the two oppositely directed light signals passonly through the Faraday sensor device along a common light path and areseparated again by optical couplers on leaving the Faraday sensor deviceas shown in International.

In two other known embodiments, two light signals pass through anoptical series connection consisting of a first optical fiber, a firstpolarizer, a Faraday sensor device, a second polarizer, and a secondoptical fiber in opposite directions. Each light signal is converted toan electric intensity signal by appropriate photoelectric transducersafter passing through the optical series connection.

In the first embodiment known from U.S. Pat. No. 4,916,387, a solidglass ring surrounding the current conductor is provided as the Faradaysensor device. The polarization axes of the two polarizers are rotatedby a 45° angle to one another. For compensation of unwanted changes inintensity in the optical lead-in fibers with this measuring arrangementwhich is known from U.S. Pat. No. 4,916,387, it is assumed that theunwanted variations in intensity (noise) and the variations in intensitydue to the Faraday effect are additively superimposed with differentsigns in the two electric intensity signals and thus can be separated.However, an indepth physical analysis leads to the result thatmechanical movements of the two optical fibers for transmitting the twolight signals essentially act as time-variable attenuation factors inthe light intensities of the two light signals. U.S. Pat. No. 4,916,387does not indicate how such different attenuation factors in the twooptical fibers can be compensated.

In the second embodiment, which is known from the Journal of LightwaveTechnology, vol. 12, no. 10, October 1994, pages 1882-1890, a fiber coilconsisting of a single-mode fiber with a low birefringence is providedas the Faraday sensor device. The polarization axes of the twopolarizers together form a polarizer angle that is different from 0°,preferably 45°. Light from a single light source is split into two lightsignals and these signals are injected into the respective optical fiberthrough an optical coupler. A measuring signal corresponding to thequotient of the difference between the two intensity signals and the sumof the two intensity signals is derived from the two electric intensitysignals that correspond to the light intensities of the respective lightsignals after passing through the series connection. Thus theattenuation factors of the two optical fibers can be essentiallycompensated. However, the light intensities of the two light signalsmust be set exactly the same when injected into the series connection.

Compensation of temperature effects on the measuring signal is notdescribed in U.S. Pat. No. 4,916,387 or in the Journal of LightwaveTechnology, vol. 12, no. 10, October 1994, pages 1882-1890. Instead,temperature-insensitive fiber coils are used as the sensor device.However, the manufacture of such fiber coils is comparativelyproblematical.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a measuring arrangementand a method of measuring a magnetic field and in particular formeasuring an electric current by measuring its magnetic field, wherevariations in intensity in the optical transmission links for twooppositely directed light signals and effects of variations intemperature are practically eliminated.

This object and others are achieved according to the present inventionas hereinafter explained. Two light signals pass in opposite directionsof travel through an optical series connection consisting of a firstoptical transmission link, a first polarizer, a Faraday sensor device, asecond polarizer and a second optical transmission link. The quotient oftwo linear functions of the light intensities of the two light signalsafter each passes through the optical series connection is determined bythe analyzing devices as the measuring signal for the magnetic field.This measuring signal is essentially independent of intensity variationsin the two optical transmission links but is generally still dependenton the temperature in the sensor device in particular. For compensationof temperature effects on the measuring signal, the polarization axis(transmission axis) of the first polarizer is then set at a firstpolarizer angle η to a natural axis (main axis) of the linearbirefringence in the Faraday sensor device and the polarization axis ofthe second polarizer is set at a second polarizer angle θ to thisnatural axis in the sensor device. Both polarizer angles η and θ areselected so they at least approximately satisfy the equation:

    cos(2η+2θ)=-2/3                                  (1)

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a measuring arrangement for measuring a magnetic field oran electric current with a Faraday sensor device.

FIG. 2 shows a measuring arrangement for measuring an electric currentwith a Faraday sensor device and special analyzing devices.

FIG. 3 shows the polarization axes of the two polarizers and a naturalaxis of the linear birefringence in the sensor device.

Corresponding parts are labeled with the same reference numbers.

DETAILED DESCRIPTION

FIGS. 1 and 2 show a Faraday sensor device 3, two optical transmissionlinks 4 and 7, two polarizers 5 and 6, a light source 10, three opticalcouplers 11, 12 and 13 and analyzing devices 20.

Faraday sensor device 3 is made of at least one material that has amagneto-optical Faraday effect. Under the Influence of a magnetic fieldH passing at least partially through sensor device 3, the polarizationof polarized light passing through sensor device 3 is altered due to theFaraday effect. Sensor device 3 may be designed in a known way with oneor more solid bodies, preferably made of glass, or with at least oneoptical fiber.

Sensor device 3 has two optical connections 3A and 3B so that lightinjected at a connection 3A or 3B passes through sensor device 3 and isoutput at the other connection 3B or 3A. The first connection 3A ofsensor device 3 is optically coupled to one end of the first opticaltransmission link 4 over the first polarizer 5. The second connection 3Bof sensor device 3 is optically coupled to one end of the second opticaltransmission link 7 over the second polarizer 6. The other end of thefirst transmission link 4 facing away from sensor device 3 is opticallyconnected to optical coupler 11 as well as analyzing devices 20 overoptical coupler 12. The other end of the second transmission link 7facing away from sensor device 3 is likewise connected optically to bothoptical coupler 11 and analyzing devices 20 over optical coupler 13.Optical coupler 11 is optically connected to light source 10 and itsplits light L of light source 10 into two light signals L1' and L2'that are sent to couplers 12 and 13 and then injected into the first andsecond transmission links 4 and 7. Both light signals L1' and L2' passthrough the optical series connection of the first transmission link 4,first polarizer 5, sensor device 3, second polarizer 6 and secondtransmission link 7 in opposite directions and are output then from theseries connection as light signals L1 and L2. Light source 10 and thethree optical couplers 11, 12 and 13 thus form the means for sending twolight signals L1 and L2 through the series connection in oppositedirections.

Couplers 11, 12 and 13 may be replaced at least in part by optical beamsplitters. In addition, instead of coupler 11 and one light source 10,two light sources may also be provided, each transmitting one lightsignal L1' and L2', respectively. The means for transmitting two lightsignals L1 and L2 through the series connection in opposite directionsmay also be formed by two photoelectric transducers that are operatedalternately as transmitter and receiver which are also provided forconverting light signals L1 and L2 to electric intensity signals afterpassing through the series connection.

The first light signal L1 is polarized linearly by the first polarizer 5after passing through the first transmission link 4 and is fed intosensor device 3 at connection 3A as linearly polarized light signal L1'.In its passage through sensor device 3, the plane of polarization of thelinearly polarized first light signal L1' is rotated by a Faradaymeasuring angle ρ that depends on magnetic field H. It is assumed that apositive angle value corresponds to the mathematically positivedirection of rotation, i.e., the counterclockwise direction, whereas anegative angle value corresponds to the mathematically negativedirection of rotation, i.e., the clockwise direction, based on thedirection of propagation of the light signal in question. The firstlight signal L1' whose plane of polarization has been rotated bymeasuring angle ρ is then sent to the second polarizer 6. The secondpolarizer 6 allows only that portion of the incoming first light signalL1' that is projected onto its polarization axis to pass through, andthus it has the function of a polarization analyzer for the first lightsignal L1'. The portion of the first light signal L1' transmitted by thesecond polarizer 6 is designated as L1 and is transmitted to analyzingdevices 20 over the second transmission link 7 and coupler 13.

The second light signal L2' first passes through the second transmissionlink 7 and is then linearly polarized by the first polarizer 5. Thesecond linearly polarized light signal L2' is injected into sensordevice 3 at connection 3A. In its passage through sensor device 3, theplane of polarization of the second linearly polarized light signal L2'is rotated by a Faraday measuring angle -ρ that depends on the magneticfield H and has the same value as that of the first light signal L1',but has the opposite sign because of the non-reciprocal property of theFaraday effect. The second light signal L2' whose plane of polarizationhas been rotated by measuring angle -ρ is then sent to the secondpolarizer 6. Second polarizer 6 allows only the component of the secondincoming light signal L2' that is projected onto its polarization axisto pass through and it thus acts as a polarization analyzer for secondlight signal L2'. The component of second light signal L2' transmittedby second polarizer 6 is designated as L2 and is transmitted toanalyzing devices 20 over the first transmission link 4 and coupler 12.

According to FIG. 3, the polarization axes (transmission axes) P1 and P2of the two polarizers 5 and 6 form an angle α that is not equal to anintegral multiple of 180° or π, based on the direction of transit oflight signal L1' or L2'. Thus polarization axes P1 and P2 of the twopolarizers 5 and 6 are not parallel.

In an especially advantageous embodiment, this angle α betweenpolarization axes P1 and P2 of the two polarizers 5 and 6 is at leastapproximately equal to +45° or -45, i.e., +π/4 or -π/4. The workingpoint for H=0 is then set in a range of optimum linearity andmeasurement sensitivity.

Light intensities I1' and I2' of the two light signals L1' and L2',before injection into the series connection, are generally set in afixed ratio to one another. The two light intensities are preferablyequal, i.e., I1'=I2'. In the embodiments illustrated here, coupler 11splits the light L of light source 10 into two equal parts with acoupling ratio of 50:50 (%).

In passing through the two transmission links 4 and 7, both lightsignals L1'/L1 and L2'/L2 undergo the same intensity variations, whichcan be caused in particular by attenuation losses due to mechanicalvibrations. These intensity variations affect light intensities I1 andI2 essentially in the form of attenuation factors. The real attenuationfactor, generally time-dependent, of an optical transmission link isdefined as the ratio of the intensity of light arriving at one end ofthe transmission link to the intensity of the light input into the otherend of the transmission link. Let A be the real attenuation factor ofthe first transmission link 4 and let B be the attenuation factor of thesecond transmission link 7. Then the following general equationsdescribe light intensities I1 and I2 of the two light signals L1 and L2after passing through the optical series connection:

    I1=I0·A·B·cos.sup.2 (ρ+α)(2)

    I2=K·I0·B·A·cos.sup.2 (ρ+α)(3).

I0 is a fixed predefined starting intensity, K is a coupling factorderived from the coupling ratios of couplers 11, 12 and 13 in theembodiment illustrated here. If the coupling ratios of all couplers 11,12 and 13 are 50:50 (%), then K=1. The cos² terms in equations (2) and(3) describe the dependence of light intensity I1 or I2 on the Faradaymeasuring angle ρ for a predefined angle α between the two polarizationaxes of the two polarizers 5 and 6. The factors in front of the cos²terms in the expressions for the two light intensities I1 and I2according to equations (2) and (3) differ only in coupling factor K.

Attenuation factors A and B of transmission links 4 and 7 are noweliminated by the fact that analyzing devices 20 derive a quotientsignal of the form:

    M=(a·I1+b·I2+c)/(d·I1+e·I2+f)(4)

Where measuring signal M for magnetic field H is derived from two linearfunctions a·I1+b·I2+c and d·I1+e·I2+f of the two light intensities I1and I2 with real coefficients a, b, c, d, e and f. At least eithercoefficients a and e or coefficients b and d are different from zero.

This measuring signal M according to equation (4) is practicallyindependent of intensity variations caused by vibrations, in particular,in transmission links 4 and 7. Thus, simple and comparativelyinexpensive telecommunications optical fibers (multimode fibers) canalso be used as transmission links 4 and 7 in all embodiments becausetheir relatively high attenuation and vibration sensitivity arecompensated in measuring signal M. However, other optical fibers or freebeam arrangements can also be used as transmission links 4 and 7.

Coefficients a, b, c, d, e and f of the linear functions in thenumerator and denominator in equation (4) may be adapted to differentinput intensities of the two light signals on injection into the seriesconnection. Coefficients a, b, c, d, e and f for light intensities I1and I2 determined according to equations (2) and (3) are preferablyadapted so as to yield a measuring signal that is essentiallyproportional to the sine of two times the Faraday measuring angle ρ

    M˜sin(2ρ)                                        (5)

without taking into account linear birefringence effects in sensordevice 3. Coefficients d, e and f of linear function d·I1+e·I2+f in thedenominator of the quotient according to equation (4) are preferably setso that linear function d·I1+e·I2+f remains practically constant and isthus independent of magnetic field H.

In a special embodiment, a quotient

    M=I1/I2=cos.sup.2 (ρ+α)/(K·cos.sup.2 (ρ-α))(6a)

or

    M=I2/I1=(K·cos.sup.2 (ρ-α))/cos.sup.2 (ρ+α)(6b)

of the two light intensities I1 and I2 is used us measuring signal M.This quotient according to equation (6a) or (6b) is derived from thegeneral quotient according to equation (4) when coefficients a=e=1 andb=c=d=f=0 or a=c=e=f=0 and b=d=1 are selected. This measuring signal Mis a relatively complex but unique function of measuring angle ρ andthus of magnetic field H.

In particular in an advantageous embodiment with at least approximatelyequal input intensities I1' and I2' of two light signals L1' and L2',the quotient

    M=(I1-I2)/(I1+I2)                                          (7)

of the difference I1-I2 (or I2-I1) and the sum I1+I2 of the two lightintensities I1 and I2 after passing through the series connection can beused as measuring signal M. This measuring signal M is then in turnproportional to sin(2ρ) if there are no linear birefringence effects insensor device 3.

Measuring signal M which has been cleared of attenuation factors A and Bof transmission links 4 and 7 can be derived by analyzing devices 20 byvarious methods from the two light intensities I1 and I2 of the twooppositely directed light signals L1 and L2. In general, each lightsignal L1 and L2 is first converted photoelectrically by analyzingdevices 20 into an electric intensity signal that is a direct measure oflight intensity I1 and I2 of light signal L1 and L2, respectively.Measuring signal M is determined from these two electric intensitysignals with the help of a table of values or by calculation. Analyzingdevices 20 contain appropriate analog or digital modules for thispurpose.

In an embodiment not illustrated here, the two electric intensitysignals are first digitized by an analog-digital converter and thedigitized signals are processed further by a microprocessor or a digitalsignal processor according to one of equations (4), (6a), (6b) or (7).

Analog components, which are usually faster than digital components, canalso be used in particular for calculation of measuring signal M as apredefined function M(I1, I2) of the two light intensities I1 and I2according to equation (4), (6a), (6b) or (7).

FIG. 2 illustrates one embodiment of the measuring arrangementcontaining analyzing devices 20 with analog components. In thisembodiment, analyzing devices 20 consist of two photoelectrictransducers 21 and 22, a subtracter 23, an adder 24 and a divider 25.First transducer 21 is optically connected to coupler 13 and convertsthe first light signal L1, after it has passed through the seriesconnection, to a first electric intensity signal S1 whose signalintensity corresponds to light intensity I1 of the first light signalL1. The second transducer 22 is optically connected to coupler 12 andconverts the second light signal L2, after it has passed through theseries connection, to a second electric intensity signal S2 as a measureof light intensity I2 of second light signal L2. Each electric intensitysignal S1 and S2 is sent to one input of subtracter 23 and one input ofadder 24. Differential signal S1-S2 (or S2-S1) at the output ofsubtracter 23 and summation signal S1+S2 at the output of adder 24 areeach sent to an input of divider 25. The output signal of the divider(S1-S2)/(S1+S2) is used as measuring signal M and is available at anoutput of analyzing devices 20. This measuring signal M thus correspondsto equation (7).

A measuring signal M that satisfies the more general equation (4) can beobtained easily with the help of analog components in an embodiment thatis not illustrated here by additionally connecting an amplifier upstreamfrom each input of subtracter 23 and adder 24, adapting the gain factorsof these amplifiers to the corresponding coefficients a, -b for negativeb, d, and e of the two linear functions in equation (4), and providingadditional adders for adding coefficient c to the output signal ofsubtracter 23 according to the numerator in equation (4) and coefficientf to the output signal at the output of adder 24 according to thedenominator in equation (4). The output signals of the two other addersare then supplied to the inputs of divider 25. If b is positive, anotheradder is preferably used instead of subtracter 23.

By adjusting coefficients a, b, c, d, e, and f for measuring signal Mformed according to equation (4), different sensitivities of twophotoelectric transducers 21 and 22 can also be compensated inparticular.

The measurement arrangement according to FIG. 2 is preferably providedfor measuring an electric current I in at least one current conductor 2.Faraday sensor device 3 detects magnetic field H that is generated bythis current I by inductance and it rotates the planes of polarizationof the two light signals L1' and L2' by a measuring angle ρ or -ρ whichdepends on magnetic field H and thus on current I. In an especiallyadvantageous embodiment illustrated in FIG. 2, sensor device 3 surroundscurrent conductor 2 so that two light signals L1' and L2' travel aroundthe current I in a practically closed light path. In this case,measuring angle ρ is directly proportional to electric current I. Sensordevice 3 may be designed as a solid glass ring with internal reflectivesurfaces that deflect light signals L1' and L2' or in any other knownmanner. Analyzing devices 20 derive a measuring signal M for electriccurrent I from light intensities I1 and I2 of two light signals L1 andL2 after they pass through the series connection, this measuring signalbeing largely independent of intensity variations in the twotransmission links 4 and 7.

Temperature effects in sensor device 3 are one problem in measuring amagnetic field H or an electric current I according Lo one of themeasuring arrangements or methods described here. These temperatureeffects induce a linear birefringence δ as a function δ(T) oftemperature T in sensor device 3, which can distort the measurement ofmagnetic field H or electric current I. Furthermore, temperaturevariations can also affect the Verdet constant and thus the measurementsensitivity.

This temperature dependence of measuring signal M is essentiallyeliminated by the measures described below for temperature compensation.Polarization axis P1 of first polarizer 5 is set at a first polarizerangle η to a natural axis (main axis, optical axis) EA of linearbirefringence δ in sensor device 3, and polarization axis P2 of thesecond polarizer 6 is set at a second polarizer angle θ to natural axisEA of linear birefringence δ in sensor device 3 (see FIG. 3). The twopolarizer angles η and θ are determined at least approximately by theequation given above

    cos(2θ+2η)=-2/3                                  (1)

A natural axis of linear birefringence δ is defined as the polarizationdirection at which linearly polarized light injected into sensor device3 leaves sensor device 3 again practically unchanged. However, iflinearly polarized light is injected into sensor device 3 with a planeof polarization not parallel to one of the natural axes of sensor device3, the light is elliptically polarized in passing through sensor device3 because of linear birefringence δ. The two natural axes of linearbirefringence δ which are generally orthogonal to one another can bedetermined in a known way. For example, sensor device 3 may be arrangedbetween a polarizer, e.g., polarizer 5, and an analyzer, e.g., polarizer6. The polarization axes of the two polarizers are set normal to oneanother. In one embodiment, the two polarization axes of the polarizerand analyzer are rotated in the same direction in relation to areference axis of sensor device 3 until the intensity of the lighttransmitted by the analyzer is zero (maximum light extinction). Thenatural axes are then parallel to the two polarization axes of thepolarizer and analyzer. As an alternative, in another embodiment the twopolarization axes may also be rotated in the same direction in relationto the reference axis of sensor device 3 until reaching the maximumintensity of the light transmitted by the analyzer (minimum lightextinction). In this case, the light is circularly polarized on leavingsensor device 3. The natural axes of linear birefringence δ are thenoffset by 45° and -45° to the polarization axis of the analyzer.

Deviations from the angle values for the two polarizer angles η and θthat exactly satisfy equation (1) are possible, in particular at a highlinear and/or circular birefringence in sensor device 3, and may amountto as much as approximately 5° in general. It follows from equation (1)in particular that polarization axes P1 and P2 of the two polarizers 5and 6 are not parallel to the natural axis EA of linear birefringence δin sensor device 3.

Angle α between the two polarization axes of the two polarizers 5 ant 6is equal to the difference η-θ or θ-η, depending on whether thedirection of passage of the first light signal L1' or the second lightsignal L2' is selected as the reference system and taking into accountthe sign of the two polarizer angles η and θ taking into account themathematical direction of rotation. The differential angle α ispreferably set so that the following equation holds at leastapproximately:

    sin(2α)=sin(±2(θ-η))=±1              (8)

Polarizer angles η and θ which satisfy this equation (8) correspond tothe above-mentioned embodiment, where differential angle α is set at itspreferred value of +45° or -45°. For example, η=10.45° and θ=55.45°, asillustrated in FIG. 3, can selected as the angle values for the twopolarizer angles η and θ that satisfy both equation (1) and (8).

Measuring signal M which is determined with polarizer angles η and θthat are set at least approximately according to equation (1)corresponds essentially to the measuring signal without linearbirefringence δ even with variable temperatures in sensor device 3,i.e., a quantity proportional to sin(2ρ) according to equation (5) inthe case of a measuring signal M derived according to equation (4) or(7).

In a special embodiment, the two polarizer angles η and θ of the twopolarizers 5 and 6 that are optimal according to equation (1) are setsimply so that in a calibration measurement, measuring signal M in itsdependence on temperature is compared with its expected theoreticalvalue without linear birefringence δ, in particular according toequation (5), for two predefined polarizer angles η and θ as parameters,and the two polarizer angles η and θ are varied until the currentmeasuring signal M matches the setpoint which is practically independentof temperature.

One advantage of temperature compensation by setting polarizer angles ηand θ is the large band width in measuring magnetic fields H or electriccurrents I. The frequency spectrum of magnetic fields H or electriccurrents I to be measured is not limited in principle by the measuresfor temperature compensation.

What is claimed is:
 1. A process for measuring a magnetic field with a sensor device that exhibits the Faraday effect, comprising the steps of:passing a first light signal in a first direction through an optical series connection consisting of a first optical transmission link, a first polarizer, the sensor device, a second polarizer and a second optical transmission link; passing a second light signal through the optical series connection in a second direction opposite to the first direction; setting a first polarization axis of the first polarizer so that the first polarization axis is rotated by a first polarizer angle in relation to a natural axis of a linear birefringence in the sensor device; setting a second polarization axis of the second polarizer so that the second polarization axis is rotated by a second polarizer angle in relation to the natural axis of the linear birefringence in the sensor device, wherein the first and second polarizer angles substantially satisfy the equation cosine(2·first polarizer angle+2·second polarizer angle)=-2/3; and determining a measuring signal for the magnetic field, the measuring signal corresponding to a first quotient of two linear functions of a pair of light intensities of the first and second light signals after each has passed through the optical series connection.
 2. The process according to claim 1, wherein the measuring signal is proportional to a second quotient of a difference between the pair of light intensities divided by a sum of the pair of light intensities.
 3. The process according to claim 1, wherein the measuring signal is proportional to a third quotient equaling one of the pair of light intensities divided by the other.
 4. The process according to claim 1, further comprising the steps of:setting a first polarizer angle of the first polarizer axis of the first polarizer in relation to the natural axis of the linear birefringence in the sensor device; and setting a second polarizer angle of the second polarizer axis of the second polarizer in relation to the natural axis of the linear birefringence in the sensor device; wherein the first and second polarizer angles substantially satisfy the equation sine(2·second polarizer angle-2·first polarizer angle)=±1.
 5. The process according to claim 1, further comprising the steps of:measuring a magnetic field of an electric current; and using the measuring signal as a measure of the electric current.
 6. An arrangement for measuring a magnetic field comprising:an optical series connection comprising a first optical transmission link, a first polarizer, a sensor device that exhibits the Faraday effect, a second polarizer and a second optical transmission link; means for transmitting a first light signal and a second light signal through the series connection so that the first light signal and the second light signal pass through the series connection in opposite directions; and at least one analyzing device for deriving a measuring signal for the magnetic field that corresponds to a first quotient equaling one of two linear functions, of a pair of light intensities of the first and second light signals after each has passed through the optical series connection, divided by the other, wherein a first polarization axis of the first polarizer is rotated by a first polarizer angle in relation to a natural axis of the linear birefringence in the sensor device and a second polarization axis of the second polarizer is rotated by a second polarizer angle in relation to the natural axis, the first and second polarizer angles substantially satisfying the equation cosine(2·first polarizer angle+2·second polarizer angle)=-2/3.
 7. The arrangement according to claim 6, wherein the first and the second optical transmission links are formed by a plurality of multimode optical fibers.
 8. The arrangement according to claim 6, wherein the at least one analyzing device derives a measuring signal that is proportional to a second quotient equaling a difference between the pair of light intensities divided by a sum of the pair of light intensities.
 9. The arrangement according to claim 6, wherein the at least one analyzing device derives a measuring signal that is proportional to a third quotient equaling one of the pair of light intensities divided by the other.
 10. The arrangement according to claim 6, wherein the first and second polarizer angles of the first and second polarization axes of the first and second polarizers are set in relation to the natural axis of the linear birefringence in the sensor device substantially according to the equation sine(2·second polarizer angle-2·first polarizer angle)=±1.
 11. The arrangement according to claim 6, wherein the sensor device is arranged in the magnetic field of an electric current and the at least one analyzing device derives the measuring signal as a measure of the electric current. 